1. Field of the Invention
The present invention relates to optics, and more particularly to microlenses.
2. Description of the Related Art
Several approaches may be used for implementing a tunable microlens. For example, one known approach is to control the refractive index of a lens using an electrostatic potential applied to that lens. Such lenses are typically referred to as gradient index (GRIN) lenses. The range of tunability (e.g., of the focal length) for GRIN lenses is governed primarily by the electro-optic coefficient of the lens material. Unfortunately, electro-optic coefficients associated with the typical materials used in GRIN lenses are relatively small. This fact results in small optical path modulation and, therefore, necessitates the use of relatively thick lenses and/or high electrostatic potentials. In addition, many electro-optic materials display strong birefringence causing the properties of a GRIN lens to be polarization dependent.
Another approach for implementing a tunable microlens is to mechanically control the shape of the microlens using flexible elastic materials, such as transparent polymers, for making the lens. A typical mechanically adjustable, flexible lens has a wider range of tunability than a GRIN lens. However for operation, mechanically adjustable lenses require external actuation devices, such as mechanical pumps, that may be laborious and expensive to implement. For example, integrating actuation devices into a two-dimensional array of tunable microlenses is particularly difficult.
Yet another approach for implementing a tunable microlens is disclosed in U.S. Pat. No. 6,014,259 to Wohlstadter, issued Jan. 11, 2000, the teachings of which are incorporated herein by reference. Wohlstadter teaches a variable-focus liquid lens controlled through self-assembled monolayers (SAMS) adsorbed on a substrate. However, one problem with such lenses is the limited choice of complementary materials that can be used for a SAM/substrate combination. Another problem is a strong hysteresis typically exhibited by SAM-controlled lenses. The hysteresis may result, for example, in failure of a lens to return to the original shape when the tuning voltage is disconnected. Furthermore, none of the above-described microlenses, including GRIN and mechanically adjustable lenses, allow for simultaneous lens position adjustment and focal length tuning.